The role of haptoglobin in phagocyte-mediated killing of Staphylococcus aureus

Haptoglobin is a positive acute-phase serum protein. It is upregulated during infection and is a valuable marker for many inflammatory-related diseases. Its primary known function is to eliminate haemoglobin from plasma to prevent loss of iron through the kidneys, thereby protecting the kidneys from damage and sequestering iron, a key target for invading bacteria. In this thesis, I show that haptoglobin also interacts with lipoteichoic acid (LTA) of Staphylococcus aureus, an important virulence factor. Bacteria are subsequently eliminated by neutrophil-, monocyte- and macrophage-mediated killing. S. aureus is an extremely common human pathogen associated with various diseases including septic arthritis and toxic-shock syndrome. A current major concern is the emergence of multidrug-resistant strains such as methicillin-resistant S. aureus (MRSA) that cause severe infections in hospitals and in the community. To increase our understanding of the defence mechanisms employed by the host against S. aureus, LTA was used to capture LTA-binding proteins from human serum and these were identified using a proteomics-based strategy. A variety of targets were captured including complement proteins, lipid-transport proteins, coagulation-cascade proteins and acute-phase proteins. Haptoglobin was selected for further study, because its expression is known to be upregulated in response to infection, and it interacts with phagocytic cells to stimulate phagocytosis. ELISA and column chromatography studies confirmed that haptoglobin binds LTA directly as well as S. aureus and suggest that the α-chain of haptoglobin is critical for this interaction. The role of haptoglobin in promoting the killing of S. aureus by neutrophils, monocytes and macrophages was evaluated by comparing in vitro bacterial-killing using serum depleted of haptoglobin. For each cell type haptoglobin was found to be a key mediator in phagocyte-mediated bacterial killing. This work highlights an important new role for haptoglobin in immune defence, and explains its upregulation during infection.